# Sociality does not predict signal complexity in response to playback in apteronotid weakly electric fishes

## TL;DR

This study finds that the complexity of electric signals in knifefish is not linked to how social they are, challenging the idea that complex social groups always need complex signals.

## Contribution

The study is the first to test the social complexity hypothesis in electric signaling, using apteronotid fish with diverse social structures.

## Key findings

- Chirp complexity and variation in apteronotid fish are not related to group size or social structure.
- Phylogenetic relatedness does not explain differences in chirp complexity across species.
- Electric signal complexity varies independently of sociality in these fish.

## Abstract

Evolution of signal structure should be influenced by the social environment in which signals are sent and received. The “social complexity hypothesis” for communicative complexity postulates that individuals living in complex social groups require complex signals to communicate effectively. This hypothesis has been supported in many vocalizing animals. In other systems and modalities, however, relationships between sociality and signal complexity are less clear. Weakly electric knifefishes represent a novel model for investigating the relationship between signal structure and social structure. South American apteronotid knifefish continuously produce quasi-sinusoidal electric organ discharges (EODs) from their tail to communicate species, sex, and status. Fish also rapidly modulate EOD frequency to produce chirps, which coordinate agonistic and courtship interactions. Sociality and chirp structure vary extensively across apteronotids. Here, we quantified chirp complexity and variation in chirp structure across six species that vary in group size: three territorial species (Apteronotus albifrons, Parapteronotus hasemani, and ‘Apteronotus’ bonapartii), two gregarious species (Adontosternarchus devenanzii and Adontosternarchus balaenops), and a semi-social species (Apteronotus leptorhynchus). Chirp complexity and variation in chirp structure differed substantially across species but were unrelated to sociality. Within the Apteronotus and Adontosternarchus species pairs, one species produced complex chirps, whereas the other produced relatively simple chirps. Thus, neither phylogenetic relatedness nor social structure explained variation in chirp structure or complexity in the species we examined.

The online version contains supplementary material available at 10.1007/s00265-025-03619-y.

The relationship between sociality and signal complexity has never been tested in the electric signaling modality. Electric fishes rely primarily on their electric sense to communicate and have extensive species variation in both social and signal structure, making them an ideal model in which to investigate how social behavior and signals covary. Apteronotid weakly electric fishes produce simple, easily quantifiable electrocommunication signals called chirps during social interactions. Group size was not associated with chirp complexity or variation in chirp parameters across six apteronotid species with a range of social structures. These findings highlight the need to investigate the relationship between social structure and signal design across multiple species and modalities to determine whether more complex social environments universally require more complex signaling regimes.

The online version contains supplementary material available at 10.1007/s00265-025-03619-y.